Is it too soon for Britain to contemplate this other part of Churchill's moral?

Not in view of the British character, the demonstrated effectiveness of British arms, and the voices - including that of US Secretary of State Alexander Haig - being raised against pressing military advantage to counterproductive lengths.

No one else, of course, can make the decisions for Prime Minister Thatcher and her advisers as to when they have gone far enough in resorting to military means to uphold international law and thwart Argentine designs. But such a military analyst as Admiral Stansfield Turner recognizes that Britain's superiority in battle presents a test of Mrs. Thatcher's statesmanship. He finds reports of ''unconditional surrender'' talk in Britain to be ominous. He sees the aftermath to be more difficult for everyone if Mrs. Thatcher does force such a surrender on the Argentines at the cost of added bloodshed.

Among British diplomats themselves there is a reported realization of the need not to risk losing future support in the world community by exacting a heavier and heavier cost in lives on both sides. Already in the eyes of many in Britain and abroad the cost has been far too high in relation to a territorial dispute that should never have brought any bloodshed in this day and age - least of all between civilized, Christian countries with considerable bonds of trade and friendship.

The Argentine generals bear a terrible burden for launching the aggression that broke off long diplomatic efforts toward resolving the situation. Britain's resolute defense of the principles of nonaggression and self-determination ensures that any magnanimity now would not be misinterpreted as weakness or abandonment of principle. Rather it would be a great nation's willingness to go the extra mile, a contribution to the United Nations' newly mandated one-week peace effort by its secretary general, Javier Perez de Cuellar.

In crass warfare terms Argentina could cut its losses by being responsive to this effort. It has shown military results against high odds. But it evidently cannot prevail by force of arms. It would lose less face by joining in a solution for the sake of the UN and international law rather than because of increased military subjection to the British.

As it is, the tragic toll mounts on both sides. Everyone agrees that a military solution is no solution, that it would have to be followed by negotiated settlement. Yet the fighting goes on. As an unflinching power with the law on its side, Britain has reason to recall Churchill's words quoted above. Its decisions for restraint or otherwise now will help determine how soon the final part of his celebrated moral is realized: